The metal coating of solar cells is understood to mean the production or application of current-discharging, electrically conductive contacts on or to the front and rear sides of solar cells. The metal coating must, therefore, be able to establish a good resistive contact with the semiconductor in order to ensure that the charge carriers emerge from the semiconductor into the current-discharging contacts without any disturbances. In order to avoid current losses, the metal coating must additionally have an adequate current conductivity, that is to say either a high conductivity and/or a sufficiently high conductor track cross section.
A multiplicity of processes which meet these requirements exist for the metal coating of rear sides of solar cells. The aim when metal coating the front side, or light incidence side, of the solar cell is to achieve the least possible shading of the active semiconductor surface, in order to use as much of the surface as possible for capturing photons. However, this can be achieved only with a smaller conductor track cross section, which conflicts with the requirements of high current conductivity. With fine conductor track structures, moreover, the metal coating may manifest adhesion problems, in particular on crystalline silicon.
Metal coating using a thick-film technique is an economical and conventional technique. The pastes used comprise metal particles (predominantly silver) and are electrically conductive as a result. They can be applied by screen, mask and pad printing or by paste writing. A widely used process at the present time is the screen printing process, by means of which finger-shaped metal coating lines having a width of up to approximately 80-100 .mu.m are possible for the metal coating. Even at this grid width, electrical conductivity losses are evident in comparison with a pure metal structure, and this can have an adverse effect on the series resistance and thus on the filling factor and the efficiency. At even smaller printed-on conductor track widths, this effect is intensified since the process causes the conductor tracks simultaneously to become flatter as well. The nonconductive oxide and/or glass components between the metal particles constitute a fundamental cause of this reduced conductivity. On the other hand, the glass component is necessary for the adhesion of the conductor tracks on the solar cell.
More complex processes for producing the front side contacts make use of a laser or photographic technique for the definition of the conductor track structures. Various metal coating steps are often necessary in order to apply the metal coating with adhesive strength and to the thickness required for the electrical conductivity. It is thus proposed in German references DE 43 11 173 A1, which is not a prior publication that is, it is not prior art to the present application, that with wet-chemical metal coating, a first, fine metal coating takes place by means of palladium nuclei and is reinforced with adhesive strength by means of currentless deposition of nickel. In order to increase the conductivity further, copper is deposited currentlessly or electrolytically over this, said copper, in turn, expediently being protected against oxidation by means of a fine silver or tin layer.
The disadvantage of these processes, which often require a plurality of metal coating baths and therefore also have a high outlay as far as disposal is concerned, is, moreover, that they have a damaging effect to a greater or lesser extent on a rear side metal coating (which is printed on, for example) and therefore the latter is often specially protected.
European references EP-A 0 542 148 discloses a process for the production of an electrode structure for the front side metal coating of a solar cell. In this case, an electrode structure is initially produced on the solar cell body using a thick-film technique and is subsequently reinforced by the electrodeposition of silver or copper.
U.S. Pat. No. 4,144,139 discloses a process for the production of a front side contact for solar cells in which a metallic base is thickened by means of currentless, light-assisted metal deposition from the solution.